In the field of motor vehicle combustion engines, there is a desire to have the ability to detect the concentration of different gaseous components in the exhaust gas stream from the engine. Such measurements can be used for controlling the operation of a combustion engine, with a view toward optimizing the amounts of injected fuel and air. If the engine can be provided with an optimal composition of the fuel/air mixture during all operating conditions, the fuel consumption and the harmful emissions from the combustion engine can be minimized.
In addition to engine control, such gas measurement should also provide the ability to be used in connection with a diagnosis of a vehicle's catalytic converter (catalyzer). In this context, the fuel and oxygen levels must lie within certain ranges in order that the vehicle's catalyzer should be able to operate optimally. A measure of the catalyzer's so-called "light-off" time, i.e. the time which elapses before the catalyzer purifies the exhaust gases optimally, can also be used during a diagnosis of the catalyzer's operation.
Different forms of gas sensors are known for achieving the above-mentioned objectives. One example of such a gas sensor, which is particularly for use in connection with motor vehicles, is the so-called lambda sensor, by means of which the oxygen content in the exhaust gases can be detected. The signal from a lambda sensor can be used in connection with optimizing the fuel and oxygen supply to the engine. In addition to the oxygen, it would be desirable to detect other components in the exhaust gases. Examples of known sensors (apart from lambda sensors) are thermistors, NO.sub.x sensors (i.e. sensors for nitrogen oxide compounds), oxygen sensors, carbon monoxide sensors and residual heat sensors.
An arrangement for detecting combustible gaseous hydrocarbons by means of a measurement bridge which has pellistors (pellet resistors) is known from British Patent No. 2,185,579. A pellistor is a resistor with a temperature-dependent resistance, as described in British Patent No. 2,044,937, for example. An application of pellistors in connection with the detection of exhaust gases in motor vehicles is described in Swedish Patent Application No. 9301715-0.
In connection with the measuring and detecting of different gas components in the exhaust gas stream from a combustion engine, a problem exists in that measurement signals from certain of the above-mentioned sensors can be influenced by other gases than those for which the sensor is intended. For example, the NO.sub.x sensor (apart from sensing the concentration of nitrogen oxide compounds) can also be sensitive to the concentration of oxygen and hydrocarbons. By using a plurality of different types of sensors at the same time, it should thus be possible to separate out each of the different gas components and, despite the cross-sensitivity of the different sensors, obtain a measurement of the composition of the measured gas. Using a plurality of different sensors in this way is, however, expensive and requires space. Each separate gas sensor requires a probe, a fixture, cabling, possibly an amplifier, an analyzer unit and a common analyzer unit which, from the signals of the different sensors, produces output signals giving the composition of the measured gas.